Light-valve projectors using liquid crystal panels as a light modulator are an attractive way to produce large screen images. Consumer or theater high definition television displays require screen sizes in excess of about 50 inches. In order to have enough brightness to overcome ambient illumination for consumer displays of this size a light output on the order of 1000 lumens is needed, while theater sized displays require roughly 5000 to 10,000 lumens. Currently available low cost light valve projectors fall short of the light output required for consumer displays, and higher priced HDTV projectors fall short of the light output required for a normal size theater. Thus, light-valve projectors using liquid crystal cells as diffractive light valves or diffraction gratings appear to be the only option for economical, bright displays of this size.
Liquid crystalline light valves for use as phase or diffraction gratings in projection systems require that the light passing through different regions of the liquid crystal cell exit the regions of the device out of phase. The phase difference between the light emerging from the device in different regions causes diffraction of the light into various orders, creating a pattern of bright spots or regions, e.g., the first, second, third order diffraction spots, and dark regions caused by the destructive interference of light emerging from different regions out of phase. Spatial filters such as louvers disposed between the viewing surface and the liquid crystal device then pass the selected higher diffraction orders of the diffraction pattern onto the projected scene, and block the zero or other unwanted orders. When the various regions of the device are divided into electrically addressable pixels, the diffraction of light through the device can be modulated to produce images.
Liquid crystalline tunable birefringence devices (TBD's) can be used to produce a diffractive light valve or diffraction grating. However, such devices require polarizers which inherently reduce the light transmission, rendering it essentially no better than a conventional twisted nematic (TN) type light valve. Twisted nematic light valve devices have also been proposed wherein the liquid crystal has a 90.degree. and 180.degree. twist across the cell. However, such devices exhibit only about a .pi./2 phase difference between ON and OFF states, such that useful efficiency can only be achieved in a reflective type display. In addition, both such approaches have the disadvantage of requiring high resolution patterned electrodes. To replace the patterned electrodes, photo-induced alignment has been considered for chirped-gratings. However, practical diffractive devices that operate at low voltages, are simple to construct, and that have high efficiencies have not been described. The transmissive modulation efficiency associated with TN cells is less than about 35%, with contrast ratios only on the order of about 100:1, and the transmissive modulation efficiency of patterned electrode devices is only about 60%, with contrast ratios on the order of 150:1. Accordingly, there is currently a need for a highly efficient liquid crystalline diffractive light valve that can be produced at low cost.